Skin creams used for cosmetic purposes, which provide protection against harmful outside agents when applied to the surface of the skin, generally contain water-repellant hydrocarbons, natural or synthetic waxes, and/or natural fats.
Natural fats usually represent mixtures of the various triglycerides whose fatty acid components contain 14, 16 or 18 carbon atoms in a straight chain.
If the fatty acids are saturated, the fats usually have a firm consistency, are not stretchable, and have only slight spreadability. If the triglycerides contain unsaturated fatty acids, the solidification point drops and the fats become more stretchable or liquid, but have the disadvantage of readily turning rancid by oxidation when exposed to the air and are therefore unsuitable for cosmetic purposes. In all cases, special preservatives must be added to preparations containing natural fats.